Carbonates with aromatic ester groups are suitable for preparing polycarbonates by the melt transesterification method, for preparing phenyl urethanes or are intermediates for active substances in the pharmaceutical and plant protection sector.
It is known that aryl carbonates may be obtained by phase interface phosgenation (Schotten-Baumann reaction) of aromatic hydroxy compounds. Here, the use of solvents and caustic soda solution is a disadvantage because partial saponification of phosgene or chloroformates can take place due to the presence of alkali. In all cases large amounts of common salt are obtained as a side product. Furthermore, care must be taken to recover the solvent.
Therefore condensation without the use of solvents in the presence of tetramethylammonium halides as catalysts has been suggested (U.S. Pat. No. 2,837,555). Here, the amounts of catalyst which are required are relatively large. In general, 5 to 7 wt. % of catalyst, with respect to the amount of phenol used, is needed in order to obtain economic rates of reaction. The reaction temperatures of 180.degree. to 215.degree. C. are linked with the risk of decomposition of the thermally labile tetramethyl ammonium halides. Furthermore, the catalyst has to be removed subsequently by washing with water, which makes its recovery much more difficult. In addition, far more than the stoichiometrically required amount of phosgene is consumed.
According to another process (U.S. Pat. No. 3,234,263), diaryl carbonates are obtained by heating phenyl chloroformates in the presence of large amounts of alkali (or alkaline earth) metal compounds using tertiary nitrogen bases as catalysts. However, this process has the disadvantage that elevated temperatures are used and the catalysts such as the alkali or alkaline earth metal compounds have to be partially dissolved in order to achieve only approximately economically acceptable reaction times. In this process half of the phosgene originally introduced is lost in the form of CO.sub.2. In addition, the chloroformates have to be synthesised in a quite separate process step.
According to CA-A-2 058 359 (U.S. Pat. No. 5,167,946), diaryl carbonates are obtained by phosgenation of aromatic hydroxy compounds in the presence of aluminium compounds which are at least partially soluble under the reaction conditions, or are converted into soluble aluminium halides and obviously act as homogeneous catalysts in this form (cf. U.S. Pat. No. 2 362 865, col. 1, 1. 45 to 53). That is the reason why aluminium trichloride (solubility) is particularly preferred. Although very good yields are obtained, it is difficult to separate the catalysts from the products. In fact, it must be reckoned that these compounds have a certain degree of volatility and that thermal decomposition may take place due to these aluminium compounds, leading to impurities, reductions in quality and decreased yields. The same applies to the process in U.S. Pat. No. 2,362,865, which still mentions the use of titanium, iron, zinc and tin as the metals or in the form of their soluble salts, particularly the chlorides and phenolates.
Thus it seemed sensible to use heterogeneous, non-soluble catalysts, which makes working up the reaction mixture a great deal easier. Proposals have also been put forward relating to this. Thus, the disclosure in EP-A-516 355 recommends in particular aluminium trifluoride, which is optionally applied to a support such as aluminosilicates. However, the synthesis of aluminium trifluoride is very complicated and expensive due to handling fluorine or hydrofluoric acid. Furthermore, metal salts on porous supports are described as catalysts for the reactions according to the invention in WO 91/06526. As can be seen from the test examples, fully continuous phosgenation of phenol on such catalysts was only possible in the gas phase, which is associated with relatively elevated reaction temperatures and the risk of decomposition of sensitive chloroformates. Obviously phosgenation of phenol with these catalysts cannot be performed in the liquid phase because the hot, liquid phenol washes out the active catalyst constituents.